This invention relates generally to drill string telemetry systems, and, more particularly, to drill string telemetry systems utilizing torsional acoustic waves for the transmission of information along a drill string, which may be in motion.
A description of the construction and operation of acoustic telemetry systems for oil wells can be found in U.S. Pat. No. 3,790,930, entitled Telemetering Systems for Oil Wells, and U.S. Pat. No. 4,001,773, entitled Acoustic Telemetry System for Oil Wells Utilizing Self Generated Noise, both of which issued in the names of the present inventor and William D. Squire and Harper J. Whitehouse as joint inventors. These patents also set forth the advantages of transmitting telemetry information from the bottom of oil well boreholes by means of acoustic waves. For the most efficient transmission in long runs of drill-string pipe, acoustic waves of the torsional type are preferred, since they sustain less energy loss at joints in the drill string, and they couple less acoustic energy into the fluid surrounding the pipe walls than do compressional waves.
As described in detail in the aforementioned patents, the frequency of torsional waves used for the transmission of telemetry data can be selected to lie in a frequency band that suffers a minimum attenuation during transmission. By way of further background, U.S. Pat. No. 3,990,827, entitled Teletering System for Oil Wells Using Reaction Modulator, also issued in the names of the present inventor and joint inventors William D. Squire and Harper J. Whitehouse, discloses an electro-acoustic transducer capable of producing acoustic waves in a drill pipe, for transmitting telemetry information, and further discloses a driving circuit that converts sensor outputs into activating signals supplied to the transducer.
It will be appreciated that a critical element in acoustic telemetry systems is a reliable acoustic wave generator. The wave generator, in turn, requires a highly reliable source of power. Self-contained power sources, such as batteries, capable of powering an acoustic wave generator for the entire life of a drilling operation are too bulky to be accommodated at the bottom of the borehole. Furthermore, supplying generator power through electrical conductors is impractical for the same reasons as telemetry using electrical conductors. A conductor located outside the drill string would be exposed to sharp rocks in the borehole, and would be likely to break or tear. On the other hand, a conductor located inside the drill string would be difficult to handle when sections of pipe were added to or removed from the drill string.
One technique for transmitting power to the bottom of the borehole is to utilize the flow of drilling mud. Drilling mud is a viscous fluid used to take into suspension drilling debris, and to remove this debris from the borehole by carrying it to the surface. The mud is pumped under pressure down the drill string pipe and down to the drill bit, where it leaves the drill string, picks up the drilling debris, and transports it to the surface.
Some telemetry devices of the prior art have used the flow of drilling mud to produce compressional acoustic waves, by inducing pressure surges in the mud flow. Other prior telemetry devices have used the flow of drilling mud to produce torsional waves by deflecting the mud flow in such a manner as to give it a tangential component of velocity. By way of example of such prior devices of the first type, U.S. Pat. No. 3,711,825 discloses a device that produces momentary pressure surges in the drilling fluid, in the nature of water hammer, by selectively closing a valve to interrupt the flow of drilling mud. The device disclosed in the aforementioned patent, in common with other prior art devices, produces waves that are chiefly of the compressional type, such waves being less effective carriers than torsional waves, especially in pipe having a long run and many joints.
U.S. Pat. No. 3,813,656 discloses two devices suitable for producing torsional waves in drill strings. In one of these, mud is selectively directed to jets, through which it is expelled tangentially to the drill string, thereby producing a rotational torque and an attendant torsional wave. Such an arrangement tends not to produce desirably sharp, nearly instantaneous torsional pulses. Further, the torque produced by the jets varies significantly with the density and viscosity of the mud.
Another technique for generating torsional waves is to employ a braking device capable of producing torsional waves by decelerating the drill string, either by engaging a normally stationary mass to the drill string, or by engaging a part of the drill bit or drill string into the sidewall of the borehole. An example of a device of the former, mass engaging type is disclosed in U.S. Pat. No. 3,813,656. When a large stationary mass is first coupled to the drill string, the mass tends to decelerate an adjacent portion of the drill string, and thereby produces a torsional wave.
Braking devices of either the mass engaging or the wall engaging type have certain inherent shortcomings. The decelerating effect, and the nature of the wave thereby generated in these devices, are dependent either upon the difference between the angular velocity of the mass and that of the drill string, in the mass engaging device, or upon the efficiency of the braking action, in the wall engaging device. The efficiency of the braking action depends, in turn, upon the coefficient of friction of the braking surfaces employed, one of which may be the sidewall of the borehole. Therefore, the decelerating effect of the brake, and the nature of the resulting wave, cannot be modified after the device is placed in service at the bottom of the borehole, and will be subject to changes in conditions in the borehole.
The changes in conditions in the borehole that affect torsional wave generation in wall engaging devices include not only changes in efficiency of the brake, but also variations in the transmission characteristics of the drill string, which variations may dictate the use of a different frequency for best wave transmission. One method of compensating for changes in braking efficiency, and for such variations in transmission characteristics that may occur as the drill bit depth increases, is to vary the rotary speed of the drill. Varying the speed in order to produce sharply defined waves can, of course, result in operation of the drill string at some speed other than the most efficient speed from a standpoint of drill bit operation. Consequently, drilling efficiency may be sacrificed to achieve satisfactory or improved data transmission.
In mass engaging devices for the generation of acoustic waves, it is possible for friction between the drill string and the mass to accelerate the mass gradually to nearly the drill string angular velocity. Such an accelerated mass when engaged would produce little decelerating effect, since the difference between the angular velocities of the mass and drill string would be small. Moreover, whenever the mass is engaged it must itself be accelerated in order to produce deceleration in the drill string. Therefore, after the mass is first used to produce a torsional acoustic wave, its angular velocity is no longer accurately known, and the nature of subsequently produced torsional acoustic waves will be largely unpredictable.
In recent years, the search for new oil sources has increasingly required deep drilling and directional, or non-vertical, drilling. Telemetry in such drilling operations is especially important, to provide operators with accurate data from the bottom of the borehole without their having to remove the drill string. As discussed above, prior acoustic telemetry techniques using mud "water hammer" rely primarily on compressional or longitudinal acoustic waves, which suffer relatively large transmission losses. Moreover, prior devices employing torsional acoustic waves generated by string drill deceleration all have significant shortcomings, as discussed in the foregoing paragraphs.
It will be apparent, therefore, that there is still a critical need for an improved acoustic wave generator for use at the bottom of a borehole; one that does not require batteries or other external electrical connections, and one capable of producing well defined torsional waves without regard to the drill string velocity or other variables. The present invention fulfills this need.